Physics
A periscope works on the principle of

Diffraction
Reflection
Total internal reflection
Refraction

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Physics
Thermohaline circulation is mainly due to

Cold, dense ocean water sinking and warmer water replacing it
Winds causing circulation of ocean water
Warm water being added to the ocean due to industial waste
Warmer, less dense water sinking in the ocean and cold water replacing it

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Physics
Superconductors are those elements

Which conduct electricity only at super-high temperatures
Whose resistance falls almost to zero at very low temperatures
Whose conductivity is intermediate between metals and insulators
Which turn into insulators at very low temperatures

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